We have an electrostatic filter on our furnace right where in main air intake connects. We recently had our ducts cleaned and they told us we also needed to have our main air intake cleaned (an extra charge) and we needed an electrostatic filter at the opening to the main air intake vent (also an extra charge). Is this true? Do we need a filter in both places?
Question of whether you can do it depends on the static pressure load your furnace is already dealing with. The nameplate on your furnace will specify the maximum static pressure allowance. This is basically a measure of how much resistance the furnace blower feels when pushing air.
Whatever you do in whatever configuration of filters including allowances for dirty filters you need to stay below this limit at all times to prevent potentially serious problems including cracked heat exchangers and frosting over of AC coils. The fancier pleated/electrostatic filters with high ratings for dust collection significantly increase overall static pressure v. cheap fiberglass.
If the guy has no clue what the static pressure of the system is and did not have a manometer to check it with both filters installed I wouldn't go there. I know my system couldn't handle it.
On question of should you.. I suppose having filters would protect return ducts from new dust not already filtered out of the air by main filter.
There will very likely be dust in your return duct work if you don't put a filter there (and there probably will still be some even if you do). This doesn't matter, since anything that gets picked up by return airflow is going to get caught by the main filter any way. You don't need a filter there, and you don't need your return cleaned.